Recently, an auto repeat request (ARQ) or hybrid auto repeat request (HARQ) scheme is widely used in order to performing effective communications by increasing throughput in a mobile communication system.
The ARQ or HARQ is a scheme whereby a receiving side transmits information on whether received data has a reception error to a transmitting side, so that the transmitting side can retransmit data having the reception error. In other words, in a system supporting the ARQ or HARQ scheme, the receiving side transmits, to the transmitting side, positive reception acknowledgement (ACK) for a packet successfully received from the transmitting side, whereas the receiving side transmits, to the transmitting side, negative reception acknowledgment (NACK) for a data packet unsuccessfully received so that the transmitting side retransmits the data packet for which NACK is received.
FIG. 1 is a diagram illustrating an HARQ scheme in a WCDMA system in accordance with a conventional art. Referring to FIG. 1, a base station determines a user equipment (UE) to which a packet is to be transmitted and a format (a coding rate, a modulation scheme, and data amount, etc) of the packet. The base station transmits control information for the packet to the user equipment on a downlink control channel (HS-SCCH) and then transmits the packet to the user equipment on a downlink data channel (HS-DSCH) at a certain time associated with the transmitting time of the control information.
After receiving the packet from the base station, the user equipment decodes the received packet. If the packet is successfully decoded, the user equipment transmits ACK to the base station through an uplink channel. The base station receiving ACK from the user equipment realizes that the packet has been successfully transmitted to the user equipment and transmits a next packet. If the user equipment fails to successfully decode the received packet, the user equipment transmits NACK to the base station. The base station receiving NACK from the user equipment configures a retransmission packet which has an identical or new packet format and transmits the retransmission packet to the user equipment. The user equipment tries to decode the retransmission data by combining the retransmission packet with the initial packet in various ways.
In adopting the ARQ to HARQ scheme, it is necessary for a transmitting side to receive ACK or NACK from a receiving side with significant reliability. In case that the transmitting side receives and decodes ACK or NACK erroneously, radio resources can be wasted by transmitting a retransmission packet which is unnecessary since the receiving side receives an initial packet successfully or by transmitting a new packet although retransmission of an initial packet is necessary since the receiving side fails to successfully receive the initial packet. For example, when a user equipment obtains NACK by erroneously receiving or decoding ACK received from a base station, the user equipment retransmits an initial packet to the base station. In this case, the base station considering that the initial packet has been successfully received may allocates, to another user equipment, radio resources which should be allocated to the user equipment. In this case, the first user equipment retransmits the initial packet and the second user equipment transmits a new packet through the identical radio resources. As a result, the base station cannot receive the retransmission packet from the first user equipment nor the new packet from the second user equipment successfully because of a data collision between the retransmission packet and the new packet on the identical radio resources.